G. H. Keller

Radiocarbon and 210Pb distribution in submersible-taken deep-sea cores from Project FAMOUS

During the FAMOUS survey of the Mid-Atlantic Ridge in August and September, 1974 by the research submersible “Alvin” two cores were taken for radiochemical analysis. One core (527-3) was 24 cm long and the other (530-4) was 17 cm long. Both were from water depths of about 2500 m. Slices of the cores were analyzed for radiocarbon and 210 Pb. In the top 8 cm layer of 527-3 dates are constant with depth at about 2400 yr B.P. Below 8 cm radiocarbon dates increase linearly yielding an accumulation rate of 2.9 cm/10 3 yr. The constant age from the surface to a depth of 8 cm can be attributed to biogenic mixing to that depth with no significant mixing below 8 cm. The excess 210 Pb pattern yields a mixing coefficient of 0.6 × 10 −8 cm 2 /sec. The top 2 cm of core 530-4 has a 14 C date of 13,000 yr B.P. Below 4 cm dates increase from 16,400 to 18,000 yr B.P., but this increase probably is not statistically significant. The data indicate physical disruption of the section. The date of this disruption is not defined by the data but the restriction of excess 210 Pb to the top centimeter of the core implies either that sediment accumulation at this site has only recently resumed or that both the rate of accumulation and rate and depth of bioturbation have been very small since the disrupting event.

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

Active slope failure, sediment collapse, and silt flows on the modern subaqueous Huanghe (Yellow River) delta

Post-depositional slope instability and bottom mass-movement processes strongly modify the progradational subaqueous slopes of the modern Huanghe (Yellow River) Delta. Wide, shallow gullies dissect the submarine slopes with gradients of 0.3 to 0.4°. Lower delta-front sediments experiencein situ subsidence, forming numerous collapse depressions. These processes are pronounced over much of the delta, incising and redistributing the most recently deposited silt-rich sediment. Principal causative factors include low sediment strengths created by rapid deposition in the delta during annual peak discharges from the river and severe bottom perturbations by surface storm-generated waves.

Suspended sediment advection by tidal currents off the Huanghe (Yellow River) delta

Studies to date indirectly indicate that only a small percentage of the sediment discharged by the Huanghe (Yellow River) is presently transported from the Gulf of Bohai to the Huanghai (Yellow Sea). Direct measurements in early summer 1985 show low concentrations of suspended sediment east of 119°45′E but high concentrations in Bohai Bay. Stokes drift associated with an amphidrome of the M2 tide may contribute to a northwestward transport of Huanghe sediment.

Organic matter and the geotechnical properties of submarine sediments

Continental slope deposits off Peru and Oregon where coastal upwelling is a pronounced oceanographic process possess significant concentrations of organic carbon. Geotechnical properties are altered to varying degrees by the organic matter. Organic matter absorbs water and causes clay-size particles to aggregate forming an open fabric. This causes unusually high water contents and plasticity and exceptionally low wet bulk densities. Some of these deposits show notable increases in shear strength, sensitivity and degree of apparent overconsolidation. Owing to the unique geotechnical properties, sediment stability characteristics are considered to be poor in situations of excess pore pressures. Failure appears to take the form of a fluidized flow somewhat similar to the quick clays of Scandinavia.

The subaqueous delta of the modern Huanghe (Yellow River)

The subaqueous delta of the Huanghe (Yellow River) has been studied using high-resolution acoustic systems. There are many subtle variations in sea floor morphology and sediment geometries; smooth, featureless areas are rare. The main components of the subaqueous delta include broad, shallow channels; moderately disturbed areas with near-surface cut and fill structures; heavily disturbed areas with sea floor depressions, pits, and gullies; and a smooth, gently sloping distal delta apron or “rise.” These features are not directly related to sediment settling from dilute surface plumes but are due to gravity-driven hyperpycnal underflows, submarine mass movements, and silt flows.

Short period internal waves over the Huanghe (Yellow River) delta front

Internal waves with periods of about 5 minutes and trough to crest heights of up to 6.2 m were observed acoustically over the actively accreting delta front of the Huanghe (Yellow River) in the western portion of the Gulf of Bohai, Peoples Republic of China. The radian frequency of the internal waves was close to the locally-observed Brunt-Vaisala frequency. Through the relatively short (∼one hour) duration of an internal wave train, the amplitude was observed to decrease progressively with time. These internal waves may cause resuspension of delta-front sediments.

Geotechnical properties of surface and near-surface deposits in the East China Sea

Abstract Shelf deposits in the East China Sea are primarily relict sands with overlying fine-grained cohesive deposits occurring along the innermost shelf and as a more or less isolated deposit on the midshelf. Considering these surface and near-surface (0 to 1.5 m) cohesive deposits as a unit, the innermost shelf sediments are slightly coarser (more silt) than those of the midshelf which commonly contain more clay-size material. The inner shelf sediments also display higher mean wet bulk densities (1.48 to 1.88 mg m−3) and shear strengths (4.0 to 9.8 kPa), but lower water contents (64 to 81%) and porosities (62 to 66%) than those found associated with the midshelf deposits. Available data indicate that the midshelf mud deposit is primarily derived from the reworking of Huanghe (Yellow River) coastal deposits that were laid down at a time when the river debouched into the Yellow Sea to the north of the Changjiang. Some portion of the midshelf mud may be derived from the Changjiang. These midshelf ‘fines’ apparently are caught up in a large circulation gyre over the shelf which accounts for their isolated nature. Strong bottom and near-bottom currents, as well as winter storm wave activity, are primary mechanisms resulting in both suspended sediment and bedload transport on the shelf of the East China Sea.